This disclosure relates to devices for the transmission of radiation, especially of light. In particular, it is related to a non-focusing reflector for the concentration of radiation such as sunlight at a desired region over a wide range of angles of incidence.
A number of systems for passive or non-tracking reflecting concentration of solar energy have been produced in the past. Among such systems are those shown in U.S. Pat. Nos. 5,537,991; 4,002,499; 4,003,638; 4,230,095; 4,387,961; 4,359,265; 5,289,356; and 6,467,916, which are all incorporated by reference in their entirety. It is appropriate to refer to the reflectors as light-transmission devices because it is immaterial whether the reflectors are concentrating radiation from a large solid angle of incidence (e.g. concentrating solar light onto a solar cell) or broadcasting radiation from a relatively small source to a relatively large solid angle (e.g. collecting light from an LED chip to form a beam).
Concentration of radiation is possible only if the projected solid angle of the radiation is increased. This requirement is the direct consequence of the law of conservation of the etendue, which is the phase space of radiation. Solar concentrators which achieve high concentration must track the sun; that is, they must continuously reorient in order to compensate for the apparent movement of the sun in an earth center (Ptolemaic) coordinate system. Reflectors, in contrast, are fixed in position for most lighting purposes. For tracking collectors the direction to the center of the sun is stationary with respect to their aperture. Such concentrators can achieve very high concentrations of about 45,000 in air. Even higher concentrations have been achieved inside transparent media.
Tracking, however, is technically demanding because solar collectors are commonly fairly large and designing these systems for orientational mobility may add significantly to their cost. Moreover, the absorber, which typically incorporates some heat transfer fluid as well as piping, also may need to be mobile. This is the motivation to study the concentration which can be achieved with stationary, non-tracking devices. The same principles apply when it is desired to deliver light or other radiant energy from a small source to a relatively large solid angle.